Some gas turbine engine fuel flow control systems are configured to controllably split total burn flow between primary (or start) fuel nozzles and secondary (or run) fuel nozzles during engine startup. In some known fuel flow control system, such as the one disclosed in U.S. Pat. No. 6,484,510, a flow divider valve is used to vary the flow split as a function of total burn flow. In other words, fuel flow to the start nozzles is controllably increased as a percentage of total flow scheduled to the engine. More specifically, as total burn flow increases during the engine start sequence, a greater percentage of flow, up to a predetermined value of total burn flow, is routed to the start fuel nozzles. When the total burn flow exceeds the predetermined value, the flow divider valve routes equal amounts of flow to the primary and secondary fuel nozzles. While this technique generally satisfies the flow spit functionality for engine startup, it can undesirably increase primary flow just after engine startup. This can result in heat induced engine combustor distress in the areas adjacent to the primary fuel nozzles.
Many fuel flow control systems, including those described above, also include an ecology valve. The ecology valve, which is connected to the primary and secondary fuel nozzles, is configured to extract a predetermined amount of fuel from the primary and secondary fuel nozzle manifolds during engine shutdown. The ecology valve is additionally configured, after a subsequent engine startup, to return the extracted fuel to the engine. In many instances, the ecology valve may be configured to return some of the fuel during the engine startup sequence. This can adversely impact the scheduled burn flow to the primary and secondary fuel nozzles, which can in turn adversely impact engine ignition.
In addition to the above, some engine applications rely on fuel staging. With fuel staging, fuel flow to certain run nozzles, sometimes referred to as staged nozzles, is blocked at lower total burn flow conditions, such as ground idle, to improve engine emissions. If fuel staging is incorporated, it is desirable to provide a means to override the fuel blockage to the staged nozzles to guard against heat induced engine combustor distress at total burn flow rates above a predetermined level.
In view of the foregoing, there is a need for a fuel flow control system that is configured to control flow to the start nozzles during engine startup to predetermined constant flow rate, with the remainder directed to the run nozzles, rather than simply increasing this flow as percentage of total burn flow. There is also a need for a fuel control system that is configured to equalize the total burn flow to the start and run fuel nozzles after engine startup. There is additionally a need for a fuel control system that is configured such that the ecology valve does not adversely impact scheduled burn flow to the primary and secondary fuel nozzles during engine startup. There is a further need for a fuel flow control system that is configured to override fuel blockage to staged nozzles. The present invention addresses one or more of these needs.